Caracterización hidrodinámica de praderas naturales de macrófitos bentónicos en fondos someros afectados por mareas

Resumen

The inner water body of Cadiz bay is a shallow and tide-dominated lagoon, affected by episodic wind-storms and profusely inhabited by marine macrophytes (> 90% covering). Benthic macrophyte distribution on this area shows a clear zonation pattern: The seagrasses Zostera noltii Hornem (intertidal) and Cymodocea nodosa Ucria (Ascherson)(intertidal and subtidal), and the rhizophytic macroalgae Caulerpa prolifera Forsskaal (Lamoroux)(subtidal) are key to the functioning and productivity of this system. Their interactions with hydrodynamics involve important ecological processes such as resources uptake and availability, bed stability and sedimentation. To understand the physical settings controlling such processes, the main objectives of this PhD thesis were: (1) to study the effects of the marine macrophytes on the spatial patterns of current velocity and turbulence. (2) to achieve their potential consequences for sedimentary and nutrients dynamics. For this propose, hydrodynamic gradients along natural canopies, as well as the influence of some benthic landscape elements (i.e. microtopography, benthic transitions and patches fragmentation) were analysed with a combination of in situ measurements (e.g. acoustic Doppler velocimeter (ADV) and dye tracer measurements) and flume tank experiments. Main results revealed that (1) the flow accelerates over the canopies, being partially explained by submergence conditions, (2) fragmentation of intertidal patches augments horizontal turbulent diffusivity of solutes, whereas their retention time remains unaffected, and (3) the leading edge is the most active sedimentation zone on a seagrass meadow, and accounts for the global sedimentation rate due to different volumetric flow rates. Microtopography seems to promote the benthic macrophyte interactions only on cases affected by high flow events, when deposition is favoured within C. nodosa beds compared to C. prolifera ones. Overall, diffusive limited- rather than mechanical stress- conditions dominate during no windy spring tide periods. Present study provides central keys to understand the role of marine macrophytes as ecosystem engineers from the inner Cadiz bay, and highlights the importance of benthic landscape elements when studying flow along this type of communities